The present invention generally relates to a diesel engine of a type having a turbulent combustion chamber and, more particularly, to the structure of the turbulent combustion chamber.
A diesel engine having, in addition to a primary combustion chamber with a piston movably supported therein, a turbulent combustion chamber communicated with the primary combustion chamber through a connecting throat is not a recent development and has long been will known in the art. In this type of diesel engine, the primary and turbulent combustion chambers are so functionally correlated that fuel oil can be injected into the turbulent combustion chamber to mix with a swirling flow of air induced therein by the introduction through the connecting throat of the air being then compressed within the primary combustion chamber. The air-fuel mixture so created within the turbulent combustion chamber is subsequently ignited by a heater plug or glow plug at the start of the engine, or by the elevated temperature of the compressed air during the normal operation of the engine, thereby producing the expanding gases, caused by the combustion, which is in turn spurted into the primary combustion chamber to drive the piston.
Since the diesel engine of the above described type requires the provision of a water-cooled jacket adjacent and exteriorly around the turbulent combustion chamber for cooling the latter, and since in the diesel engine of the above described type the increased temperature of the compressed air is apt to decrease as it flows at high velocity from the primary combustion chamber into the turbulent combustion chamber throttled through the connecting throat, the temperature of the wall defining the turbulent combustion chamber may remain relatively low at the time of start of the engine where the ambient temperature is very low such as during the winter season, and will not readily increase to a required value even during the subsequent warm-up of the engine. The consequence is that fuel oil injected into the turbulent combustion chamber will not be sufficiently vaporized and, therefore, will not be mixed uniformly with the air swirling within the turbulent combustion chamber, thereby posing problems associated with the difficulty to ignite the mixture at the time of start of the engine and the failure to continue the firing of the mixture subsequent to the start of the engine. As is well known to those skilled in the art, these problems in turn result in the emission of not only unburned exhaust gases, but also a white smoke consisting of steam accompanied by the unburned exhaust gases.
In an attempt to obviate the above described problems which often arise during the start and the subsequent warm-up of the engine, a method has been employed to advance the fuel injection timing for a predetermined degree to increase the delay period, or ignition lag, i.e., the period from the beginning of the fuel injection to the moment of ignition so that the time increment can be utilized for facilitating the mixing of fuel with air within the turbulent combustion chamber. As a matter of practice, the advancement of the ignition timing depends on the cetane number of the fuel used and the ambient temperature in which the engine is operated, and, for example, in the North America Continent where the weather is relatively cold and the fuel of small cetane number is widely used, the fuel injection timing is advanced a relatively great value.
It has, however, been found that the above discussed method to advance the fuel injection timing for the purpose of minimizing the diesel knocking results in the increased generation of noises. Specifically, the advancement of the fuel injection timing to increase the delay period means that a correspondingly increased amount of fuel air is injected before the actual ignition thereof and the resultant mixture of fuel oil with air is ignited at several points within the turbulent combustion chamber, accompanied by a sudden, great pressure rise which in turn result in shaking of the cylinder wall and the cranking mechanism thereby constituting a cause of the noise generation.
Apart from the above discussed method, another method for the improvement of the engine starting performance has been suggested, for example, in Japanese Laid-open Patent Publication No. 54-151709, first published Nov. 29, 1979, wherein there is employed, as shown in FIG. 1 of the accompanying drawings, the connecting throat 3 having its effective cross-sectional surface area gradually increasing from its opening confronting the primary combustion chamber 1 towards its opposite opening confronting the turbulent combustion chamber 2 such that, during the compression stroke, the air entering the turbulent combustion chamber 2 to create a swirling air a can be retarded to avoid both the reduction in temperature of the air within the turbulent combustion chamber which would occur in heat-exchange with the swirling air and the blow-off of once-ignited fuel oil which may be caused by the strong swirling flow of the air and that, during the subsequent expansion stroke, the ignited mixture directed towards the primary combustion chamber can, as it flows through the connecting throat 3, be substantially throttled to increase the velocity energy of the ignited mixture thereby to improve the combustibility of the mixture within the primary combustion chamber 1.
Although the above mentioned publication is silent as to the direction in which the fuel oil is injected from the fuel injection nozzle 5, the drawings employed therein clearly illustrate that the fuel oil f injected from the nozzle 5, the position and the angle of which are generally determined in dependence on those of the glow plug 4, is shown as traveling so as to impinge upon a side wall 9 defined in an insert 8 having a flat-bottomed recess with the flat bottom 7 lying in parallel to the undersurface 6 of the cylinder head. In this prior art structure, it has been found that, although the starting and warm-up performance of the engine can be somewhat improved, the mixing of the injected fuel oil with the swirling air a does not take place favorably because the injected fuel oil impinging upon the side wall 9 is then scattered in the same direction as the direction of travel of the swirling air a, forming a film of fuel oil on the side wall 9 and the flat bottom 7. Therefore, the prior art structure is still far from solving the problem associated with the failure to ignite the fuel oil and, if the problem is desired to be solved without the starting and warm-up performance being adversely affected, the injection timing has to be advanced. As hereinbefore described, the advancement of the injection timing brings about the problem associated with the noise generation.
Moreover, in the prior art structure of the turbulent combustion chamber shown in FIG. 1, the angle .theta. is relatively small between the plane of the flat bottom 7 and the plane in which that portion of the wall defining the connecting throat, which is situated on one side adjacent the longitudinal axis of the primary combustion chamber 1 lies. The employment of the relatively small angle .theta. has been found disadvantageous in that the swirling air a, that is, the combustion gases of the mixture ignited within the turbulent combustion chamber can not be smoothly guided towards the primary combustion chamber through the connecting throat 3 and, therefore, the stream of the combustion gases flowing toward the primary combustion chamber 1 through the connecting throat 3 can not be intensified. In addition, another disadvantage has also been found that no good heat dispersion take place at the acute-angled portion represented by the angle .theta. and, since the acute-angled portion is apt to be heated by the fuel oil being injected, the heat load is high at that acute-angled portion.
In order to obviate the problems as hereinbefore described, it may be contemplated to increase the angle .theta.. If the angle .theta. is increased, the connecting throat would be directed downwards, resulting in the possibility that the expanding gases caused by the combustion within the turbulent combustion chamber and subsequently flowing through the connecting throat impinge upon the top face of the piston with the velocity thereof consequently reduced. This may in turn result in that the expanding gas entering the primary combustion chamber would not be uniformly distributed all over the primary combustion chamber and the remainder of the air within the primary combustion chamber would not be fully utilized, with the diesel engine tending to emit a relatively large amount of smoke particularly during a high load operating condition.
Separate from the employment of the connecting throat having its effective cross-sectional surface area gradually increasing towards the turbulent combustion chamber as hereinbefore discussed, Japanese Laid-open Utility Model Publication No. 52-8907, published in 1977, discloses the turbulent combustion chamber having its flat bottom inclined downwards so as to converge with the undersurface of the cylinder head at an angle within the range of 5.degree. to 60.degree., while the connecting throat has its longitudinal axis inclined upwards at an angle within the range of 30.degree. to 45.degree. relative to the undersurface of the cylinder head so as to intersect with the inclined plane of the flat bottom at an angle not less than 45.degree.. The purpose of the design disclosed in the last mentioned publication is described to minimize the heat load evolved on the acute-angled portion between the wall defining the connecting throat and the flat bottom thereby to increase the durability of the cylinder head. This last-mentioned publication neither discloses nor suggests at all the improvement in engine starting performance, the minimization of the smoke emission, and the maximization of the use of the air within the primary combustion chamber, and this is evidenced by the facts that the last mentioned publication is silent as to the relationship between the fuel injection nozzle and the angle of the inclination of the flat bottom, that the connecting throat is shown as having a uniform effective cross-sectional surface area all over the entire length thereof, and that the numerical limitations for the various angles disclosed therein include the arrangement wherein the connecting throat may extend substantially perpendicular to the top face of the piston.